What started as a simple task to add test coverage ended up giving way to a light refactoring. This consolidates the mostly-identical `CMerkleBlock` constructors into one (using C++11 constructor delegation) and adds coverage for the by-txids construction case.
### Before
![selection_006](https://user-images.githubusercontent.com/73197/30242104-0f381fe4-9545-11e7-9617-83b87fce0456.png)
### After
![selection_008](https://user-images.githubusercontent.com/73197/30242107-1425dfaa-9545-11e7-9e6b-2c3432517dd1.png)